In What Settings Does Enough Near-Surface Cooling Occur to Form Fog?įog can also form where moist air is cooled enough to reach its dew-point temperature. In both cases, evaporation of precipitation is causing the precipitation fog. Rain that reaches the ground can also evaporate, increasing the humidity of the near-surface air, potentially forming more fog. Since cold air has a lower water-vapor capacity than warm air, the increase in humidity can cause the cold air to reach its dew point, forming fog and other low-level clouds. If the underlying colder air is dry, it can cause some or all of the falling raindrops to evaporate, increasing the humidity of the cold air. The released rain then begins to fall through the underlying cold air and toward the cold surface.ĥ. As the moist, warm air rises over the cold air, it is cooled and can reach saturation, forming clouds and rain. Along a warm front, warmer, less dense air is forced to rise at a gentle angle over more dense, colder air.Ĥ. A warm front is a mass of warm air that moves laterally, trying to displace cold air in its path. Fog can also form in association with precipitation, such as occurs along a warm front. is the Tule fog of the Central Valley of California, the white area in the center of this regional satellite image.ģ. Perhaps the most famous valley fog in the U.S. If moisture is added to the valley air, such as from wetlands, irrigated fields, canals, and lakes, the humidified and cooled air can reach the dew point, forming a valley fog. In rugged terrain, cold nocturnal (nighttime) downslope winds can trap atmospheric moisture in valleys. Valley Fog Produced by Increased HumidityĢ. This phenomenon is common in fall, when water bodies are likely to remain warm from the long summer, but air above them begins to cool. The instability causes an evaporation fog to rise in vertical columns, as shown in the photograph to the left. The increase in humidity and cooling causes the rising air to reach saturation, forming a type of fog called an evaporation fog. As the unstable air rises, it can incorporate water molecules that evaporate from the water body, increasing the humidity of the air. When a warm water body underlies colder air, the environmental lapse rate is steep and the atmosphere is unstable. Addition of moisture to the ambient air may happen when a nearby source of water for evaporation exists, or when precipitation evaporates as it falls through drier air. What Conditions Can Add Enough Humidity to the Air to Form Fog?įog can form where enough moisture is added to the air to cause saturation (100% relative humidity), but more commonly fog forms where the addition of moisture occurs simultaneously with the chilling of the air. One way in which air can reach saturation is by increasing the moisture content until vapor pressure reaches the watervapor capacity, moving up on this graph.Īnother way to reach saturation is by cooling the air until it reaches the dew-point (or frost-point) temperature, represented on this graph as moving to the left.Ī combination of cooling and increasing humidity can also cause saturation. It is most common along coastlines, over large water bodies, and in certain low areas that have the right setting to produce fog, but fog can form nearly anywhere, under the right conditions.įog indicates that the relative humidity has reached 100% and the air temperature has reached the dew-point (or frost-point) temperature, causing saturation. What Is Fog?įog is a ground-level cloud that obscures visibility, such as with this lighthouse. Fog forms in several distinct settings, each of which causes air at or near ground level to become saturated. Specifically, fog is produced by cooling of the air, by increasing the humidity of the air, or some combination, with the end result that water vapor content in the air reaches saturation. FOG IS SIMPLY A CLOUD at ground level, so the same conditions that create a cloud also produce fog.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |